With the progress of technology, electronic devices are pursued to be miniaturized and keep performances improved in the interim. Therefore, technique of fin field effect transistor (FinFET) is introduced to improve performances of traditional field effect transistor (traditional FET). A fin field effect transistor has a fin-shaped structure to form a non-planar double-gate transistor built on a silicon-on-insulator (SOI) substrate, unlike a traditional field effect transistor with a planar structure. The fin-shaped double-gate structure makes it possible to have two electrically independent control gates in order to enhance the flexibility of electrical designs and produce devices with higher efficiency and lower electric consumption. The tendency of pursuing miniaturization and high performances makes FinFET technology to be a main trend of future electronic industry, however, the known fabricating methods and the device performances thereof still needs to be improved.